1. Field of the Invention
The field of the invention is that of the optical devices used to measure the orientation of an object in space without contact. There are various possible fields of application, but the main application is the detection of aircraft pilot headset posture, thus making it possible to project into his or her visor an image exactly superposed on the outside landscape or to slave various systems of the apparatus to his or her gaze. The posture of an object should be understood to be its orientation and its position relative to a known frame of reference.
2. Description of the Prior Art
There are various optical techniques that can be used to measure orientation on a headset. Generally, noteworthy elements are installed on the headset which are identified by an optical emission and reception system. The position of the images of these noteworthy elements makes it possible to determine, by computation, the position and the orientation of the headset.
To this end it is possible to use retroreflecting cube-corners or retroreflectors. All that is required is to arrange the optical emission and reception members on a same axis. These retroreflector systems are, by nature largely insensitive to solar lighting.
By way of exemplary embodiment, they can be combined with a fixed opto-electronic device comprising a spot source associated with an assembly comprising one or two matrix sensors without optical lens. In this arrangement, the reflector is equipped with a mask applied onto its input face. This mask comprises a transparent central part and an opaque peripheral part. The contour of the mask is in the form of a polygon, thus embodying at least the orientation of two fixed directions of the headset. The orientation of the headset is computed by the analysis of the forms of the contour projected onto the sensor or sensors. The analysis focuses on the transitions between the light and dark areas of the reflection received by the sensor.
To determine the orientation and the position of the reflector, the only things used are either the projections of the sides of the contour of the polygonal mask, or, in the absence of mask, the projections of the sides of the polygonal contour of the reflector.
Each side of the contour provides, by central projection onto a plane, two concurrent directions and therefore a leak point.
At the periphery of the angular field, only two sides of the contour are effectively projected. They are two consecutive sides of the contour, and they provide, on the plane, four concurrent directions in pairs, i.e. two leak points.
In the central part of the angular field, at least three sides of the contour are effectively projected. They provide, on the plane, at least six concurrent directions in pairs, i.e. three leak points.
FIGS. 1 to 8 illustrate this phenomenon of the number of leak points in the simplified case of a reflector with equilateral triangular contour;
FIG. 1 represents the general principle of the projection onto a plane P1 of a point M1 of the contour of the mask of the reflector Re of vertex O, from the radiation of the spot source S, close to P1. The point T is the intersection of the input face of the reflector and of the straight line SO. On P1, the projection of T common with that of O is the point T.
FIG. 2 represents the input face of a cube-corner reflector without mask, with equilateral triangular input face PQR.
FIGS. 3 and 4 represent, for a spot lighting source at infinity, that is to say a source S far from the reflector, the reflection represented by shading obtained by projection onto an image sensor close to the source S, in the particular case where this projection plane is parallel to the input face of the reflector. Two configurations arise. The first configuration is represented in FIG. 3. The incident radiation is close to the edge of the angular field of the reflector. The contour is a parallelogram. The second configuration is represented in FIG. 4. The incident radiation is then close to the centre of the angular field. The contour is a hexagon with sides that are parallel in pairs. These contours are both centred on the projection T′ of T, the centre of symmetry between the projected symmetrical triangles P′-Q′-R′ and P′0-Q′0-R′0.
When the projection plane is not parallel to the plane of the input face PQR, the reflections are distorted, but, for each of the reflection contours, their sides remain parallel in pairs. The vertices and the sides of each reflection are symmetrical relative to the point T′, the projection of the point T.
When the source S is at a finite distance, the sides of the contour of the polygonal reflections are no longer parallel to one another, the reflection no longer has a centre of symmetry but T′ remains the point of conjunction of the diagonals from the opposite vertices of the contour of the reflection and the intersections of the opposite sides provide, respectively, on the projection plane which corresponds to the plane of the sensor, two leak points at the edge of the angular field as can be seen in FIG. 5 and three leak points in the central angular field as can be seen in FIG. 6.
The reduction of the number of leak points from three to two by moving from the central field to the field edge is, of course, as indicated above, also valid in the case of a reflector with polygonal contour or with polygonal mask contour which may be planar or not.
Consequently, whatever the polygonal contour, other than the mask in parallelogram form, to determine the six orientation/position unknowns, only four parameters corresponding to the coordinates of two leak points are measured at the field edge. Thus, at the field edge, the orientation/position measurement is difficult to perform, without the help either of a fixed ancillary device in proximity to the source such as, for example, a blanking screen or a resist in a return mirror, or of a second sensor. Such is the first drawback of mask posture detection systems according to the prior art.
Also, a cube-corner reflector exhibits an ambiguity of orientation, with nothing to a priori distinguish the different sides of the mask. A form coding produced by means of markings in the form of nicks or bosses is therefore applied onto the contour of the reflector or of its mask in proximity to the vertices. In this way, the projection of this coding makes it possible to unequivocally associate each of the vertices of the projected figure with one of the vertices of the contour of the reflector or of its mask. The production of the different marks and their identification constitutes the second drawback of the mask posture detection systems according to the prior art.
Finally, the angular field of a cube-corner reflector is limited to a solid angle of π/2 sr. This field is insufficient to cover, for example, the head rotations of a user. It is therefore essential to be able to use several cube-corners of different orientation to cover a greater field. Such is the third drawback of the mask posture detection systems according to the prior art.